A technique is known, in which, when the temperature of exhaust gas of an internal combustion engine is low, ammonium nitrate produced by the reaction of ammonia and NOx is held in a selective reduction type NOx catalyst (hereinafter, referred to as an SCR catalyst), thereby suppressing emission of NOx into the air (for example, see PTL 1).
On the other hand, when the temperature of the SCR catalyst or the temperature of exhaust gas is low, it may be difficult to supply ammonia to the SCR catalyst. In this case, it is not possible to increase the amount of ammonia adsorbed on the SCR catalyst. When ammonium nitrate is produced, ammonia is consumed. Accordingly, the amount of ammonia adsorbed on the SCR catalyst decreases. When the temperature of the SCR catalyst is sufficiently increased and the SCR catalyst is activated, if the amount of ammonia adsorbed on the SCR catalyst is small, the amount of ammonia necessary for reducing NOx may be insufficient. If the amount of produced ammonium nitrate is large, even when the temperature of the SCR catalyst is increased, it takes a lot of time until ammonium nitrate is removed. Even when the temperature of the SCR catalyst is increased to a temperature sufficient for reducing NOx, ammonia cannot be adsorbed at a place where ammonium nitrate is present. For this reason, it becomes difficult to purify NOx. Accordingly, while it is possible to suppress the outflow of NOx when producing ammonium nitrate, thereafter, when the temperature of the SCR catalyst is sufficiently increased, NOx flows out due to an insufficient amount of ammonia, and the outflow amount of NOx may be increased as a whole.